The invention relates to methods of making diagnostic and therapeutic radiology equipment. In particular, the invention relates to recrystallized tungsten-rhenium cathode filaments and cathode filament recrystallization methods. The invention also relates to improved methods for making cathode assemblies and filaments used in x-ray generating equipment, such as, but not limited to, computerized axial tomography (C.A.T.) scanners.
X-rays are produced in a vacuum as electrons are released, accelerated, and then abruptly decelerated or stopped in the target of an x-ray tube. To release electrons, a cathode filament is heated to incandescence (white heat) by passing an electric current through it. The electrons are accelerated by a high voltage, for example in a range from about ten thousand to greater than hundreds of thousands of volts, between the anode (positive) and the cathode (negative). The electrons then impinge on the anode, where they are abruptly slowed down or stopped.
Alignment is important for both x-ray tube focusing and focal spot definition in an x-ray tube. It is important to initially align the filament in the cathode cup assembly during manufacture, and maintain its alignment throughout the manufacturing cycle and operation of the x-ray tube.
Composition, grain microstructure, and recrystallization methods influence a cathode filament's creep behavior, ductility, electron emission, and alignment in an x-ray tube. Cathode filaments are often formed from tungsten materials, such as wires comprising potassium-doped tungsten. The cathode filaments are formed into a desired filament configuration. For example, the cathode filament is formed from a wire with a diameter in a range from about 0.22 mm to about 0.29 mm, for example about 0.25 mm, and formed into a coil having an external diameter of about 0.9 mm. The cathode filament also comprises legs or straight sections that are used for attaching the cathode filament to a cathode cup assembly. The cathode filaments are prepared for x-ray use by a recrystallizing heat treatment, which creates a grain structure that provides creep resistance and promotes ductility and facilitates electron emission.
Cathode filaments have been previously recrystallized after placement in a cathode cup assembly. The cathode filament's legs are inserted into the cathode cup assembly with the coil supported by the stationary legs alone. The recrystallization method comprises passing current through the cathode filament that causes resistive heating (also known as "flashing") of the cathode filament to a temperature of about 2800.degree. C. The resistive heating recrystallizes the coil of the cathode filament, however the legs are not recrystallized due to their positioning in the cathode cup assembly. During the recrystallization, the heated filament, especially at the coil, is subjected to gravitational stresses. The cathode filament will sag and distort in the coil area. Further, when the cathode filament expands upon heating, expansion is constrained by the stationary legs, thus generating stresses and creep strains. Therefore, the cathode filament is moved out of alignment in the x-ray tube.
In resistive heating recrystallization, the cathode filament's temperature is a function of the wire's diameter and the current passed through the cathode filament. Since wire diameters inevitably vary, recrystallization temperatures will vary and can not be accurately controlled. The current carrying capacity of the cathode filament is reduced by leads and welds that are used to attach the cathode filament to the cathode cup assembly. Thus, the cathode filament may not carry sufficient current to be heated to the desired recrystallization temperature, and total recrystallization of the cathode filament will not be achieved.
A cathode filament that has been recrystallized by resistive heating typically becomes mis-aligned as a result of creep deformation. These cathode filaments require re-alignment, re-seating, and re-heating in the cathode cup assembly to provide proper alignment. These re-seating, re-aligning, and re-heating steps may need to be repeated, in some instances up to five times, until a proper alignment is attained. These repeated steps are inefficient, uneconomical, and undesirable.
Therefore, it is desirable to provide a recrystallized cathode filament that maintains its alignment throughout manufacture and use in an x-ray tube. It is also desirable to provide a method for recrystallizing a cathode filament without reducing its low temperature ductility and requiring re-seating, re-aligning, and re-heating steps, as in resistive heating.